Readers have the option to return audiobooks on Audible, Amazon or iTunes for a refund, which will decrease the number of units you have sold and therefore your royalties. This means that if the book is purchased at the end of January, but isn’t returned until February, then the unit sold will appear in the January sales report, while the return will appear as negative royalties in the February month report.
This is a policy of the major audiobook retailers, and is not something Tellwell has any control over.